This invention relates to a microwave evanescent-mode oscillator comprising a length of waveguide having a cut-off frequency higher than the working frequency of the oscillator together with a Gunn element disposed within the waveguide. Such oscillators are discussed in a communication entitled "Investigation of waveguide-below-cutoff resonators for solid-state active devices" by Ivanek, Shyam and Reddi in Electronic Letters dated May 15, 1969 at pages 214 to 216.
The object of the present invention is to provide an improved device.